A direct access storage device. See floppy disk, hard disk, magnetic disk, optical disc, CD-ROM and DVD.
On the disk, data are stored in sectors, which hold a chunk of data (typically 512 bytes) and are the smallest unit that can be read or written. Memory is broken up into squares like a checkerboard, each square holding one byte. The contents of any single byte or group of bytes can be calculated, compared and copied independently. That is how fields are put together to form records and broken apart when read back in.
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